Semiconductor switching devices of a power converter, such as an inverter, perform switching operations based on drive signals which undergo pulse width modulation (PWM) with the carrier frequency being in the range of several KHz to about twenty KHz. During the switching operations, switching noises having frequency components of several dozens of KHz are generated from the power converter.
Of the frequency components of the switching noises as described above, those components having a frequency of one hundred KHz or higher have adverse influences on external equipment. In view of this situation, various legal regulations have been laid on power converters in recent years, in an attempt to reduce or eliminate such adverse influences. To deal with the regulations, the power converters are equipped with suitable noise-cut filters.
A known example of noise-cut filter for a power converter includes a single reactor in which an electric wire is wound around a core formed of, for example, ferrite, amorphous alloy, or crystalline alloy, and a single condenser in the form of a film or a chip. The reactor and the condenser are coupled into an inverted L-shape, to provide a filter for filtering switching noises that occur upon switching operations of semiconductor switching device of the power converter.
As one of the prior-art references concerning noise-cut filters for power converters, "Integrated Output Filter and Diode Snubber for Switchmode Power Converters", IEEE, 1994, pp. 1240-1245 discloses a filter circuit having an integrated, flat-sheet structure including a rectifier, RC snubber circuit, and an LC filter circuit. This reference also provides explanation of basic methods for manufacturing the respective circuits.
In addition, "Integrated Filters For Switch-Mode Power Supplies", IEEE, 1995, pp. 809-816 as another prior-art reference discloses three types of LC filter circuits having difference structures depending upon dielectric materials used therein. Namely, the dielectric materials are classified into those having a ceramic structure, and those having a non-ceramic structure, and one type of filter circuit (using BaTiO.sub.3 as dielectric material) having a flat-sheet structure is proposed as of a type using a ceramic material as a dielectric, while two types of filter circuits (sheet type, and vapor deposition type using plasma) having a film-like structure are proposed as of a type using a non-ceramic material as a dielectric.
The reactor used in the above-described known type of noise-cut filter generally has a toroidal shape, and the capacitor is a pin-inserted type and has a flat shape or a cylindrical shape. Where the reactor and capacitor are mounted on a printed board inside the power converter, a space required for mounting these components will be greater than the sum of the volumes of the respective components, resulting in a reduced assembling efficiency. Where this type of noise-cut filter is mounted on the printed board by separate wiring, the number of coupling portions will be increased, and the manner of fixing the individual components will be undesirably complicated.
A so-called composite LC filter of a chip type or pin-inserted type, which is commercially available and includes a composite unit of inductor and capacitor, has a cut-off frequency of several MHz or higher, whereas the cut-off frequency normally required to filter switching noises occurring upon switching operations of semiconductor switching devices is as low as about 150 KHz, for example. Thus, the commercially available filters are unable to filter the switching noises occurring upon switching operations of the semiconductor switching devices.
Furthermore, in a power converter such as an inverter, several amperes of current is desired or required to flow through a noise-cut filter used for the converter. It is thus difficult to employ the commercially available composite LC filter as the noise-cut filter, in view of the current capacity and the cost.
The filter circuits disclosed in the above-identified two prior-art references have problems of complicated manufacturing methods, which make the circuits unsuitable for practical use and result in increased manufacturing cost.